It's very important that you're not mediocre in absolutely everything. There should be at least one field you know in depth. As far as I know, this is the only way anyone ever gets a good feel for what it means to know anything in depth. It's much better to know two or more things in depth. Then the generalizations you make about deep knowledge in areas where you're not an expert will be operating on more data.

It's also common to know one thing in depth, but then fail to realize what this implies about every other field of knowledge. It's not a good thing, IMO.